Wastewater treatment method and method for recovering low-molecular-weight valuable substance

Abstract

The purpose of the present invention is to provide: a wastewater treatment method whereby a low-molecular-weight valuable substance can be selectively separated and made to have a high purity; and a method for recovering a low-molecular-weight valuable substance using the treatment method. This wastewater treatment method includes a step in which wastewater containing a low-molecular-weight valuable substance having a molecular weight less than 100 and cations is separated with an NF membrane. The wastewater treatment method is characterized in that the NF membrane, when used to treat an aqueous MgSO4 solution having a concentration of 2,000 mg / L at 25°C and an operation pressure of 0.76 MPa, has an SO4 2- rejection of 90% or higher and characterized by including a step in which a sulfuric acid ion component is added to the wastewater.

Description

Wastewater treatment method and low molecular weight valuable material recovery method

[0001] The present invention relates to a method for treating wastewater in which wastewater containing low-molecular-weight valuable substances is separated using an NF membrane, and a method for recovering low-molecular-weight valuable substances using the same.

[0002] For example, wastewater generated in the plating and metal processing industries may contain low-molecular-weight valuable substances such as boron, and depending on the state of the wastewater, boron may be present in the wastewater in the form of boric acid or the like. For this reason, various methods for recovering and reusing elements such as boron and compounds containing boron, among the low-molecular-weight valuable substances contained in wastewater, have been investigated. Known recovery and reuse methods include, for example, coagulation and sedimentation, resin adsorption, dry concentration, and a method combining the dry concentration method with crystallization. However, from the viewpoint of reducing the amount of chemicals used and energy consumption, more efficient treatment methods have been desired.

[0003] Known energy-efficient wastewater treatment methods include those using various separation membranes and those using separation membranes as a pretreatment step for various wastewater treatment steps. For example, Patent Document 1 discloses a method for treating wastewater containing sulfate ions and boron, which includes a first step of removing sulfate ions from the wastewater and a second step of contacting the wastewater from which the sulfate ions have been removed with a layered double hydroxide to remove boron by adsorption onto the layered double hydroxide, and which uses a reverse osmosis membrane or a nanofiltration (NF) membrane to remove the sulfate ions in the first step.

[0004] JP 2015-144997 A

[0005] However, in the treatment method of Patent Document 1, although a reverse osmosis membrane or a nanofiltration membrane is used as a separation membrane for separating sulfate ions and boron, no measures are taken to prevent permeation of alkali metals when a nanofiltration membrane is used. In other words, there is no process for increasing the concentration of sulfate ions in the wastewater, and therefore, when a nanofiltration membrane is used, it is not possible to increase the selective permeability of low-molecular-weight valuables that permeate the separation membrane by sulfate ions.

[0006] Therefore, an object of the present invention is to provide a method for treating wastewater that can selectively separate and highly purify low-molecular-weight valuable substances, and a method for recovering low-molecular-weight valuable substances using the same.

[0007] As a result of extensive research aimed at solving the above problems, the present inventors have discovered that, when wastewater containing low-molecular-weight valuables is subjected to membrane separation using an NF membrane, increasing the concentration of sulfate ions in the feed solution can suppress the permeation of cations such as alkali metals, thereby increasing the selective permeability of the low-molecular-weight valuables, and have thus completed the present invention. That is, the present invention includes the following aspects.

[0008] [1] A method for treating wastewater, comprising a step of separating wastewater containing low-molecular-weight valuables and cations having a molecular weight of less than 100 using an NF membrane, wherein the NF membrane is a membrane containing MgSO 4 having a concentration of 2000 mg / L. ４ SO when the aqueous solution was treated at 25°C under an operating pressure of 0.76 MPa ４ ２－ a rejection rate of 90% or more of the above-mentioned sulphate ions is 90% or more; and the method for treating wastewater comprises a step of adding a sulphate ion component to the wastewater.

[0009] According to the wastewater treatment method of the present invention, the NF membrane is used, so that low molecular weight valuable substances with a molecular weight of less than 100 can easily pass through, while the SO ４ ２－ Since the rejection rate of SO is over 90%, ４ ２－ Ions tend to pass through the NF membrane in an electrically neutral state, i.e., in the form of counterions, and the addition of sulfate ions reduces the amount of SO in the feed solution. ４ ２－ As the cations increase, they form counter ions with cations such as alkali metals, and the number of cations that permeate the NF membrane decreases relatively, which is thought to increase the rejection rate of cations such as alkali metals. Cations such as alkali metals naturally permeate the NF membrane easily, and as a result of this increased rejection rate, the wastewater treatment method of the present invention is able to selectively separate low-molecular-weight valuable substances and achieve high purification.

[0010] [2] The method for treating wastewater according to [1], wherein the wastewater contains monovalent anions, and the ratio of the equivalents of sulfate ions to the equivalents of the monovalent anions in the feed wastewater supplied to the NF membrane is greater than 30.

[0011] As described above, ions tend to pass through the NF membrane in the form of counterions, and an increase in the amount of monovalent anions that form counterions with cations such as alkali metals results in a relative increase in the number of cations such as alkali metals that pass through the NF membrane, which tends to reduce the rejection of cations such as alkali metals. Therefore, the greater the ratio of the equivalent of sulfate ions to the equivalent of monovalent anions in the feed wastewater, the higher the rejection of cations such as alkali metals due to the balance between the two. When the equivalent ratio exceeds 30, low-molecular-weight valuables can be more selectively separated and highly purified.

[0012] [3] The method for treating wastewater according to [1] or [2], wherein the low-molecular-weight valuable substance is one or more selected from the group consisting of boron, boron-containing compounds, deuterium, deuterium-containing compounds, organic acids, and alcohols.

[0013] When these compounds have a molecular weight of less than 100, they easily permeate NF membranes and are desirable as low molecular weight valuables to be purified from the viewpoints of treating various types of wastewater and recovering low molecular weight valuables.

[0014] [4] The method for treating wastewater according to any one of [1] to [3], wherein an aqueous solution of sulfuric acid is added when adding the sulfate ion component.

[0015] An aqueous solution of sulfuric acid generates protons, which are hydrogen cations. As the number of protons increases, the amount of monovalent anions that form counter ions with alkali metal cations also increases, thereby further reducing the rejection rate of alkali metal cations by the NF membrane and improving the selective separation of low-molecular-weight valuables.

[0016] [5] A method for recovering low-molecular-weight valuables, comprising a step of separating wastewater containing low-molecular-weight valuables and cations having a molecular weight of less than 100 using an NF membrane and recovering the low-molecular-weight valuables from the permeate, wherein the NF membrane is a membrane containing MgSO 4 having a concentration of 2000 mg / L. ４SO when the aqueous solution was treated at 25°C under an operating pressure of 0.76 MPa ４ ２－ a rejection rate of 90% or more; and a method for recovering low-molecular-weight valuable substances, the method comprising the step of adding a sulfate ion component to the wastewater.

[0017] According to the method for recovering low molecular weight valuables of the present invention, it is possible to perform wastewater treatment that can selectively separate and highly purify low molecular weight valuables as described above, and therefore it is possible to recover highly purified low molecular weight valuables as a permeate, which leads to energy savings and simplification of the subsequent processes.

[0018] [6] The method for recovering low-molecular-weight valuables according to [1], wherein the wastewater contains monovalent anions, and the feed wastewater supplied to the NF membrane has a ratio of sulfate ion equivalents to the monovalent anion equivalents of more than 30.

[0019] According to this recovery method, as described above, the greater the ratio of the equivalent of sulfate ions to the equivalent of monovalent anions in the supply wastewater, the higher the rejection rate of cations such as alkali metals due to the balance between the two. When the equivalent ratio exceeds 30, low-molecular-weight valuables can be more selectively separated and highly purified.

[0020] [7] The method for recovering low-molecular-weight valuables according to [5] or [6], wherein the step of recovering low-molecular-weight valuables includes a concentration step by evaporating water.

[0021] When a concentration step by evaporation of water is included, the method for recovering low molecular weight valuables of the present invention, which can selectively separate and highly purify low molecular weight valuables, is effective, particularly from the viewpoint of energy saving.

[0022] According to the present invention, it is possible to provide a wastewater treatment method capable of selectively separating and highly purifying low molecular weight valuable substances, and a method for recovering low molecular weight valuable substances using the same.

[0023] The present invention relates to a method for treating wastewater, a method for recovering low molecular weight valuables, and a spiral membrane element for treating wastewater.

[0024] Hereinafter, an embodiment of the present invention will be described.

[0025] [Wastewater Treatment Method] (Wastewater to be Treated) The wastewater treatment method of the present invention includes a step of separating wastewater using an NF membrane, and uses wastewater containing low-molecular-weight valuables and alkali metals having a molecular weight of less than 100 as the wastewater to be treated. The wastewater to be treated may have been subjected to a pretreatment step using a UF membrane (ultrafiltration membrane), an MF membrane (microfiltration membrane), or the like to remove impurities, silt, clays, fungi, algae, and colloidal suspended solids.

[0026] Low-molecular-weight valuable materials contained in wastewater include compounds with a molecular weight of less than 100, elements that can be recovered, or compounds containing such elements. The elements or compounds may be dissolved in the wastewater as is, or in the form of ions, etc. Examples of such low-molecular-weight valuable materials include one or more selected from the group consisting of boron, boron-containing compounds, deuterium, deuterium-containing compounds, organic acids, and alcohols. Examples of organic acids include aliphatic carboxylic acids such as acetic acid, formic acid, and propionic acid, and sulfonic acids such as methanesulfonic acid. Examples of alcohols include ethanol, methanol, propanol, and butanol. Examples of boron-containing compounds include boric acids such as orthoboric acid and metaboric acid, metal salts of boric acid, and boron halides. Examples of deuterium-containing compounds include compounds in which one or more hydrogen atoms in these compounds have been replaced with deuterium. Among these, boron is particularly preferred as a target low-molecular-weight valuable substance, since it is contained in large amounts in wastewater generated in the plating and metal processing industries, and various recovery methods are being investigated.

[0027] The concentration of low molecular weight valuable substances contained in the wastewater to be treated is, for example, about 100 to 10,000 mg / L, but a high concentration is preferable from the viewpoint of concentration and valuable recovery after membrane treatment.

[0028] Examples of cations contained in wastewater include monovalent, divalent, or higher valent cations. The present invention is particularly effective for wastewater containing monovalent cations because it can enhance the rejection performance of monovalent cations. Examples of monovalent cations include alkali metal ions, ammonium ions, and oxonium ions. Examples of alkali metals include one or more of sodium, potassium, and lithium. Alkali metals exist in wastewater as cations, but may also include those present as fine particles. The concentration of cations or alkali metals contained in the wastewater to be treated is, for example, 100 to 10,000 mg / L, and is preferably 50,000 mg / L or less from the viewpoint of maintaining a certain level of alkali metal ion rejection by the NF membrane and from the viewpoint of membrane treatment osmotic pressure.

[0029] The wastewater may also contain monovalent anions, and examples of the monovalent anions contained in the wastewater include one or more anions of halogen atoms such as fluorine, chlorine, bromine and iodine, and nitrate ions.

[0030] From the viewpoint of enhancing the effect of adding sulfate ion components, the concentration of monovalent anions contained in the wastewater to be treated is preferably 1,000 mg / L or less, and more preferably 100 mg / L or less. The lower the concentration of monovalent anions, the better, but the lower limit of the concentration is, for example, 1 mg / L.

[0031] The wastewater to be treated may also contain divalent metal cations, divalent anions, and the like.

[0032] (Separation Step) The wastewater treatment method of the present invention includes a step of separating the wastewater as described above using an NF membrane and a step of adding sulfate ion components to the wastewater. Such a wastewater treatment method can be carried out using, for example, a membrane separation apparatus as shown in Figure 1.

[0033] For example, the membrane separation apparatus shown in FIG. 1 includes a membrane module M1 having a separation membrane 1 which is an NF membrane, a supply section for a feed liquid 7, a discharge section for a permeate liquid 8, and a discharge section for a concentrated liquid 9, and is provided with a chemical supply tank 16 in the line for the feed liquid 7 for supplying an aqueous sulfuric acid solution 17 which is a sulfate ion component.

[0034] Such a membrane separation apparatus may be equipped with other devices such as pumps, sensors, tanks, control valves, and control devices as necessary, and configured to operate under desired conditions. Also, a line for circulating a part or all of the concentrate 9 to the feed liquid 7 may be provided.

[0035] The sulfate ion component is SO ４ ２－ Any compound containing the above may be used, and examples thereof include sulfuric acid, sulfates, and hydrogen sulfates. However, sulfuric acid is preferred from the viewpoint of further reducing the rejection rate of alkali metal cations by the NF membrane and enhancing the selective separation of low-molecular-weight valuables. When adding a sulfate ion component, it is preferable to use a liquid such as an aqueous solution from the viewpoint of ease of handling.

[0036] From the viewpoint of ease of handling, the concentration of the sulfate ion component to be added when added to wastewater is preferably 1 to 90 mass %, more preferably 15 to 75 mass %.

[0037] In order to enhance the selective separation of low-molecular-weight valuables, the feed wastewater to which sulfate ion components are added and which is then supplied to the NF membrane preferably has a ratio of the equivalent of sulfate ions to the equivalent of monovalent anions of more than 30, more preferably at least 40. Furthermore, if this equivalent ratio becomes too large, the corresponding effect decreases, so if the equivalent ratio is around 100, sufficient separation performance can be obtained.

[0038] Considering this point and the concentration of monovalent anions in general wastewater, the concentration of sulfate ions in the feed wastewater to which the sulfate ion component has been added is preferably 1,000 to 50,000 mg / L, more preferably 2,000 to 10,000 mg / L.

[0039] The operating conditions of the membrane separation device, such as operating pressure, permeate recovery rate, and operating temperature, can be the same as those of a general NF membrane. However, from the viewpoint of stable continuous use, the membrane flux should be 5 to 40 LMH (L m －２ ・h －１ It is preferable to adjust the operating pressure so that

[0040] (NF membrane) As the NF membrane (nanofiltration membrane), MgSO with a concentration of 2000 mg / L was used. ４ SO when the aqueous solution was treated at 25°C under an operating pressure of 0.76 MPa ４ ２－ The SO 2 is preferably used in an aqueous solution having a rejection rate of 90% or more, more preferably 95% or more, more preferably 98% or more, and most preferably 99% or more. ４ ２－ The higher the rejection rate, the greater the SO content in the permeate. ４ ２－ Furthermore, the rejection rate of cations such as alkali metal ions can be increased. ４ ２－ The rejection rate is specifically a value measured by the method described in the Examples.

[0041] On the other hand, from the viewpoint of increasing the permeability of the NF membrane to low-molecular-weight valuable substances and their recovery rate, the molecular weight cutoff of the NF membrane is preferably 50 to 200 daltons, more preferably 80 to 120 daltons.

[0042] The molecular weight cutoff of an NF membrane is measured as follows: First, a plurality of polyethylene glycols having different average molecular weights and monodisperse molecular weight distributions are prepared. An aqueous solution containing one of the plurality of polyethylene glycols at a concentration of 5000 ppm is dissolved in water at a temperature of 25°C and a pressure of 4 kg / cm. 2 The polyethylene glycol is supplied to the membrane surface of the NF membrane under the conditions of . This allows the rejection rate of polyethylene glycol to be measured. The rejection rates of other polyethylene glycols are measured using the same method. A fractionation curve showing the relationship between the obtained rejection rate and the average molecular weight of the polyethylene glycol is created. Based on the fractionation curve, the average molecular weight of the polyethylene glycol at which the rejection rate is 90% is determined. The determined average molecular weight can be considered the molecular weight cutoff of the NF membrane.

[0043] The NF membrane may be, for example, a composite semipermeable membrane comprising a porous support membrane and a separation functional layer, with the separation functional layer being supported by the porous support membrane. The material and structure of the porous support membrane are not particularly limited. For example, an ultrafiltration membrane in which a microporous layer having an average pore size of 0.01 to 0.4 μm is formed on a nonwoven fabric is used as the porous support membrane. Examples of materials for forming the microporous layer include polysulfone, polyarylethersulfone such as polyethersulfone, polyimide, polyvinylidene fluoride, and polytetrafluoroethylene.

[0044] NF membranes are classified into charged and uncharged types depending on whether or not they have a surface charge, and either type can be used in the present invention. However, when the wastewater contains a small amount of organic compounds, a negatively charged NF membrane is preferred.

[0045] An example of a negatively charged NF membrane is one having a separation functional layer with an anionic group, such as a sulfonic acid group or a carboxylic acid group, with the sulfonic acid group being a strong acid group being preferred.

[0046] Resins constituting the separation functional layer include polysulfone-based resins, polyamides, cellulose acetate, and polyvinyl alcohol, with polysulfone-based resins being particularly preferred from the viewpoint of chemical, mechanical, and thermal stability. Examples of polysulfone-based resins include polysulfone, polyethersulfone, and polyphenylsulfone.

[0047] That is, a preferred separation functional layer of an NF membrane is one containing a polysulfone-based resin having sulfonic acid groups. In particular, an NF membrane having such a separation functional layer has higher durability against alkaline cleaning solutions and chlorine-based cleaning solutions.

[0048] As the NF membrane having a separation functional layer made of sulfonated polyethersulfone having a negative fixed charge, those described in JP-A Nos. 61-4505 and 61-4506 are particularly preferred.

[0049] Examples of polysulfone-based resins having sulfonic acid groups include those having the following repeating units (A) or (B).

[0050]

[0051]

[0052] The membrane module M1 using an NF membrane can be composed of one or more membrane elements. The membrane element is typically a spiral-wound membrane element using an NF membrane. The membrane module M1 may be composed of a pressure vessel and one or more spiral-wound membrane elements disposed inside the pressure vessel. However, the structure of the membrane element containing the NF membrane is not limited to the spiral type, and may be other types such as a hollow fiber type, a tubular type, or a frame and plate type.

[0053] (Spiral-Wound Membrane Element) As shown in Fig. 3, for example, a spiral-wound membrane element comprises a perforated central tube 5 and a wound body R containing a separation membrane 1 wound around the central tube 5. In the example shown in Fig. 3, the element comprises a plurality of membrane leaves L, each having a permeate-side channel material 3 interposed between opposing separation membranes 1, a feed-side channel material 2 interposed between the membrane leaves L, a perforated central tube 5 around which the membrane leaves L and the feed-side channel material 2 are wound, and a plug 12 that prevents mixing of the feed-side channel and the permeate-side channel. In this case, the permeate-side channel within the membrane leaf L can be formed by a permeate-side channel material 3 (also referred to as a permeate-side spacer).

[0054] 3 shows an example in which the sealed portion includes both end sealing portions and an outer peripheral sealing portion 12. Of the sealed portions, the both end sealing portions are formed by sealing two edge portions on both sides of the membrane leaf L in the axial direction A1 with an adhesive. The outer peripheral sealing portion 12 is formed by sealing the edge portion of the outer peripheral tip of the membrane leaf L with an adhesive. The area surrounded by the opposing separation membrane 1, the both end sealing portions, and the outer peripheral sealing portion 12 forms a permeate side flow path, which is structured to communicate with the opening 5a of the central tube 5. A first end member 10 having a function such as a seal carrier may be provided on the upstream side of the membrane element wound body R, and a second end member 20 having a function such as an anti-telescope member may be provided on the downstream side.

[0055] When the membrane element is in use, it is housed in a pressure vessel, and a feed liquid 7 is supplied from one end of the membrane element. The supplied feed liquid 7 flows along the feed-side flow path material 2 in a direction parallel to the axial direction A1 of the central tube 5, and is discharged from the other end of the membrane element as a concentrated liquid 9. In addition, permeated liquid 8, which has permeated through the separation membrane 1 while the feed liquid 7 flows along the feed-side flow path material 2, flows along the permeation-side flow path material 3, then flows through the openings 5a into the central tube 5, and is discharged from the end of the central tube 5.

[0056] [Pretreatment Step] In the present invention, prior to the separation step using an NF membrane, it is also possible to perform pretreatment steps such as removing solids, reducing monovalent anions, softening water, and removing soluble organic impurities that are not subject to recovery. For example, by subjecting the wastewater to be treated to membrane separation using a UF membrane (ultrafiltration membrane) or an MF membrane (microfiltration membrane), it is possible to remove solids such as impurities, colloidal suspensions, monosaccharide amino acids, and soluble polymer compounds. Here, a UF membrane refers to a membrane with an average pore size of approximately 0.001 μm to 0.01 μm. A MF membrane refers to a membrane with an average pore size of approximately 0.01 μm to 10 μm.

[0057] The material of the UF membrane or MF membrane is not particularly limited, and polymeric materials such as cellulose ester polymers such as cellulose acetate, polyethylene, polypropylene, polysulfone, polyvinylidene fluoride, and polyethersulfone can be used. From the viewpoint of durability and washability, polyvinylidene fluoride and polyethersulfone are preferred. The shape of the UF membrane or MF membrane is not particularly limited, and can be selected from flat membranes, hollow fiber membranes, pleated membranes, tubular membranes, and the like.

[0058] In addition, when the concentration of monovalent anions in the wastewater to be treated is high, it may be effective to reduce the concentration of monovalent anions by using an ion exchange device having an ion exchange membrane, ion exchange resin, or the like.

[0059] Examples of anion exchange resins (negative ion exchange resins) include strongly basic anion exchange resins having a quaternary amine as a functional group and weakly basic anion exchange resins having a primary to tertiary amine, and examples of anion exchange membranes include those having a similar chemical structure.

[0060] In particular, strongly basic anion exchange resins can be used in a wide pH range and can adsorb a variety of anions, such as SO ４ ２－ >I － >NO ３ － >Br － >Cl － When the wastewater to be treated contains sulfate ions, the sulfate ions are also adsorbed onto the anion exchange resin. In the separation step, sulfate ion components can be added to the feed wastewater to compensate for the adsorbed and reduced sulfate ions.

[0061] [Method for recovering low molecular weight valuables] The method for recovering low molecular weight valuables of the present invention is a method for recovering low molecular weight valuables, comprising the steps of separating wastewater containing low molecular weight valuables having a molecular weight of less than 100 and alkali metals using an NF membrane, and recovering the low molecular weight valuables from the permeate, wherein the NF membrane is a 2000 mg / L MgSO ４ SO when the aqueous solution was treated at 25°C under an operating pressure of 0.76 MPa ４ ２－ The method for recovering low-molecular-weight valuables of the present invention utilizes the wastewater treatment method of the present invention, and the separation step, pretreatment step, etc. are as described above.

[0062] The method for recovering low molecular weight valuables from the permeate can be carried out by a combination of a step of further concentrating the low molecular weight valuables in the permeate, a step of crystallizing (or reactive crystallizing) the low molecular weight valuables in the permeate, a step of separating the solid low molecular weight valuables from the liquid, a step of drying the low molecular weight valuables, and the like.

[0063] For the concentration treatment, heating evaporation, reduced pressure evaporation, membrane separation using an RO membrane, etc. can be used, for the crystallization treatment, cooling crystallization, reduced pressure crystallization, reactive crystallization, etc. can be used, and for the solid-liquid separation treatment, centrifugation, filter separation, etc. can be used.

[0064] In addition, when the low molecular weight valuables are liquid at room temperature (for example, alcohol), the low molecular weight valuables can be recovered with high purity by combining the distillation apparatus with processes such as absorption, dehydration, and membrane separation (VP method).

[0065] The method for recovering low-molecular-weight valuables of the present invention can be carried out, for example, using an apparatus as shown in Fig. 2. In this apparatus, the permeate obtained by carrying out the wastewater treatment method of the present invention is heated to evaporate water to further concentrate the low-molecular-weight valuables in the permeate, and then cooled to crystallize the low-molecular-weight valuables in the permeate. The crystals of the low-molecular-weight valuables are separated from the liquid, and the obtained low-molecular-weight valuables are dried, thereby recovering the low-molecular-weight valuables.

[0066] The present invention will be described below with reference to examples, but the present invention is not limited to these examples. In the examples, physical properties were measured or evaluated by the following methods. The physical properties in the present invention are specifically values ​​measured by the following methods.

[0067] (1) Special Officer ４ ２－ The rejection rate was measured in accordance with JIS K3805 (1990) by the following method: ４ An aqueous solution (pH 6.5 to 7) was used to permeate an NF membrane of a predetermined size at an operating pressure of 0.76 MPa and a temperature of 25°C. After the 30-minute warm-up period, the SO content of the permeate and feed solution was measured using an ion concentration measuring device (Dionex ICS-6000, manufactured by Thermo Fisher Scientific) using ion chromatography. ４ ２－ The concentration (mg / L) was measured. From the results, the SO ４ ２－ The inhibition rate was calculated.

[0068] SO ４２－ Rejection rate (%) = (1 - (SO of permeate) ４ ２－ Concentration / SO of feed solution ４ ２－ (concentration) × 100 (2) Rejection of low molecular weight valuable substances As in (1), an aqueous solution of low molecular weight valuable substances with a concentration of 2000 mg / L was used and permeated through an NF membrane (PRO-XS3, manufactured by Nitto Denko) of a predetermined size at an operating pressure of 0.76 MPa and a temperature of 25°C. After the completion of a 30-minute preparation phase, the concentrations (mg / L) of the low molecular weight valuable substances in the permeate and feed liquid were measured using a concentration measuring device (Nexis GC-2030, manufactured by Shimadzu Corporation) using gas chromatography. From the results, the rejection of the low molecular weight valuable substances was calculated based on the following formula.

[0069] Rejection rate (%) of low molecular weight valuable substances = (1 - (concentration of low molecular weight valuable substances in permeate liquid / concentration of low molecular weight valuable substances in feed liquid)) x 100

[0070] (3) Concentration of Various Ions Contained in Feed Wastewater The feed wastewater (without sulfuric acid added and after sulfuric acid added) to the NF membrane was used as a sample to measure the concentration of various ions (mg / L) using an ion concentration measuring device (Dionex ICS-6000, manufactured by Thermo Fisher Scientific) by ion chromatography. For metal ions and boron, an ICP-MS (ICP-Mass Spectrometry) measuring device (ICPS-7510, manufactured by Shimadzu Corporation) was used. The boric acid concentration was calculated by converting the measured boron concentration using the following formula: Boric acid concentration = Boron concentration ÷ 0.175. Therefore, even when the boron concentration is used directly to calculate the rejection rate, the rejection rate of boron will be the same as the rejection rate of boric acid.

[0071] (4) Rejection of various ions The feed wastewater was passed through the NF membrane at an operating pressure of 1.2 MPa and a temperature of 25°C, and after a 30-minute preparatory period, the concentrations (mg / L) of various ions in the permeate were measured using the same measuring device as in (3). From the results and the results in (3), the rejection of various ions was calculated based on the following formula:

[0072] Rejection rate of low molecular weight valuable substance (%)=(1−(ion concentration of permeate liquid / ion concentration of feed wastewater))×100 (Experimental Example 1) Using the compounds shown in Table 1 as low molecular weight valuable substances, the rejection rate of low molecular weight valuable substances by the NF membrane was determined. The results are shown in Table 1 together with the molecular weight.

[0073]

[0074] From this result, SO ４ ２－ Even when using an NF membrane (PRO-XS3, manufactured by Nitto Denko) with a rejection rate of 99% or more, it was found that low molecular weight valuable substances with a molecular weight of less than 100 can be passed through the permeate with a high recovery rate (low rejection rate). ４ ２－ It is believed that by using an NF membrane with a rejection rate of 90% or more, low molecular weight valuable substances with a molecular weight of less than 100 can be highly purified with a high recovery rate.

[0075] (Example 1) PRO-XS3 manufactured by Nitto Denko was used as the NF membrane. ４ ２－ When the rejection rate of SO ４ ２－ The rejection rate was 99.7%. ４ ２－ The wastewater used was a model wastewater containing boric acid that may be generated in various processes, and had the composition shown in Table 5 (Comparative Example 1).

[0076] Using a membrane separation apparatus such as that shown in Figure 1, 4 mL of industrial sulfuric acid (75% sulfuric acid aqueous solution) was added to 1 L of this wastewater to prepare a feed wastewater with the composition shown in Table 2. This was supplied to the NF membrane at the same operating pressure and a temperature of 25°C, and the concentrate and permeate were discharged. After 30 minutes, a sample was taken from the permeate, and the concentration of each component in the permeate was measured as described above to determine the rejection. The results are shown in Table 2 together with the difference in rejection and the anion equivalent ratio.

[0077]

[0078] As shown in the results in Table 2, by adding sulfate ion components to wastewater, it was possible to obtain boric acid-containing water with high purity and low concentrations of Na and K.

[0079] Here, the negative value of the rejection rate of iodine anions indicates that the concentration of iodine anions in the permeate is higher than that in the feed solution. Ions tend to permeate in an electrically neutral state, i.e., in the form of counterions, when passing through an NF membrane. The addition of sulfuric acid increases the hydrogen ions on the feed side, and the amount of permeated iodine anions, which form counterions with these hydrogen ions, also increases. As a result, the concentration of iodine anions in the permeate increases.

[0080] And, Na, which forms a counter ion with the iodine anion ＋ The molar amount of hydrogen ions increases compared to cations such as Na ＋ It is considered that the permeation amount of cations such as hydrogen ions and sodium ions decreases, and the rejection rate of cations increases. In other words, the greater the equivalent of sulfuric acid that generates hydrogen ions, and the smaller the equivalent of monovalent anions that form counter ions with the cations, the more the permeation of the cations can be suppressed. The greater the ratio of the equivalent of sulfate ions to the equivalent of monovalent anions, the more the Na ＋ The rejection rate of cations such as ammonium hydroxide and ammonium hydroxide can be increased, thereby increasing the purity of low molecular weight valuable substances.

[0081] (Example 2) Membrane separation was performed using feed wastewater having the composition shown in Table 3, which was prepared by adding 10 mL of a 75% aqueous sulfuric acid solution to 1 L of wastewater in Example 1. Samples were taken from the permeate and the rejection of each component was determined under the same conditions as in Example 1. The results are shown in Table 3 together with the difference in rejection and the anion equivalent ratio.

[0082]

[0083] As shown in the results in Table 3, by adding sulfate ion components to wastewater, it was possible to obtain boric acid-containing water with high purity and low concentrations of Na and K.

[0084] (Example 3) Membrane separation was performed using feed wastewater having the composition shown in Table 4, which was prepared by adding 2 mL of a 75% aqueous sulfuric acid solution to 1 L of wastewater in Example 1. Samples were taken from the permeate and the rejection of each component was determined under the same conditions as in Example 1. The results are shown in Table 4 together with the difference in rejection and the anion equivalent ratio.

[0085]

[0086] As shown in the results in Table 4, by adding sulfate ion components to wastewater, it was possible to obtain boric acid-containing water of high purity with lower concentrations of Na and K compared to when no sulfate ion components were added. However, in order to increase the rejection rate of each of Na and K ions to 90% or more, it is necessary to reduce the concentration of monovalent anions (e.g., I － It has been found that a ratio of equivalents of sulfate ions to equivalents of HCl greater than 30 is effective.

[0087] Comparative Example 1 Samples were taken from the permeate and the rejection of each component was determined under the same conditions as in Example 1, except that membrane separation was performed using wastewater having the composition shown in Table 5 to which no aqueous sulfuric acid solution was added. The results are shown in Table 5 together with the difference in rejection and the anion equivalent ratio.

[0088]

[0089] As shown in the results in Table 5, when sulfate ion components were not added to the wastewater, the rejection rates of Na and K decreased, making it difficult to obtain highly pure boric acid-containing water.

[0090] (Comparative Example 2) PRO-XS3 manufactured by Nitto Denko was used as the NF membrane. ４ ２－ at a concentration of 400 mg / L, Cl － A wastewater containing the compound at a concentration of 2500 mg / L and having the composition shown in Table 6 was prepared.

[0091] Using a membrane separation apparatus such as that shown in Figure 1, this feed wastewater was fed to an NF membrane at a temperature of 25°C, and the concentrate and permeate were discharged. After 30 minutes, a sample was taken from the permeate, and the concentrations of each component in the permeate were measured as described above to determine the rejection. The results are shown in Table 6 together with the difference in rejection and the anion equivalent ratio.

[0092]

[0093] As shown in the results in Table 6, Cl － Even when wastewater containing boric acid is used and no sulfate ion components are added to the wastewater, the rejection rate of Na and K decreases, making it difficult to obtain highly pure boric acid-containing water.

[0094] (Reference Example 1) PRO-XS3 manufactured by Nitto Denko was used as the NF membrane. ４ ２－ at a concentration of 10,400 mg / L, Cl － A wastewater containing the compound at a concentration of 750 mg / L and having the composition shown in Table 7 was prepared.

[0095] Using a membrane separation apparatus such as that shown in Figure 1, this feed wastewater was fed to an NF membrane at a temperature of 25°C, and the concentrate and permeate were discharged. After 30 minutes, a sample was taken from the permeate, and the concentrations of each component in the permeate were measured as described above to determine the rejection. The results are shown in Table 7 together with the difference in rejection and the anion equivalent ratio.

[0096]

[0097] As shown in the results in Table 7, Cl － When sulfate ion components were added to wastewater containing sodium and potassium, the rejection rate of monovalent anions (e.g., Cl) was improved. － Since the ratio of the equivalent of sulfate ions to the equivalent of potassium was low, the rejection rate of Na and K did not reach 90% or more.

[0098] According to the present invention, there is provided a method for treating wastewater that can selectively separate and highly purify low-molecular-weight valuables such as boric acid. Because highly purified low-molecular-weight valuables can be recovered as the permeate, energy savings and process simplification can be achieved in downstream processes, making the method particularly useful as a method for recovering low-molecular-weight valuables.

[0099] 1 Separation membrane (NF membrane) 7 Feed liquid (supplied wastewater) 8 Permeate liquid 9 Concentrate 16 Chemical liquid supply tank 17 Sulfuric acid aqueous solution (sulfate ion component) M1 Membrane module

Claims

1. A method for treating wastewater comprising a step of separating wastewater containing low molecular weight valuables and cations having a molecular weight of less than 100 using an NF membrane, the NF membrane being a 2000 mg / L MgSO ４ SO when the aqueous solution was treated at 25°C under an operating pressure of 0.76 MPa ４ ２－ a sulfate ion component added to the wastewater, the sulfate ion component having a rejection rate of 90% or more.

2. A method for treating wastewater as described in claim 1, wherein the wastewater contains monovalent anions and the feed wastewater supplied to the NF membrane has a ratio of the equivalents of sulfate ions to the equivalents of the monovalent anions that exceeds 30.

3. The method for treating wastewater as described in claim 1, wherein the low molecular weight valuables are one or more selected from the group consisting of boron, boron-containing compounds, deuterium, deuterium-containing compounds, organic acids, and alcohols.

4. The method for treating wastewater according to claim 1, wherein an aqueous solution of sulfuric acid is added when the sulfate ion component is added.

5. A method for recovering low molecular weight valuables, comprising a step of separating wastewater containing low molecular weight valuables and cations having a molecular weight of less than 100 using an NF membrane and recovering the low molecular weight valuables from the permeate, the NF membrane being configured to separate low molecular weight valuables having a molecular weight of less than 100 using an NF membrane having a concentration of 2000 mg / L. ４ SO when the aqueous solution was treated at 25°C under an operating pressure of 0.76 MPa ４ ２－ a rejection rate of 90% or more; and a method for recovering low molecular weight valuables comprising the step of adding a sulfate ion component to the wastewater.

6. A method for recovering low molecular weight valuables as described in claim 5, wherein the wastewater contains monovalent anions and the feed wastewater supplied to the NF membrane has a ratio of the equivalents of sulfate ions to the equivalents of the monovalent anions that exceeds 30.

7. The method for recovering low molecular weight valuables according to claim 5, wherein the step of recovering the low molecular weight valuables includes a concentration step by evaporating water.

Images